The present invention relates to the field of transmission over optical fiber, and more particularly to the field of wavelength division multiplexed (WDM) transmission using a dispersion-shifted line fiber.
The refractive index profile of an optical fiber is generally described as a function of the appearance of the graph of the function that associates the refractive index with the radius of the fiber. Conventionally, distance r to the center of the fiber is plotted along the x-axis, and difference between refractive index and the refractive index of the cladding of the fiber is plotted up the y-axis. Thus, the terms xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d, or xe2x80x9ctrianglexe2x80x9d index profiles are used for graphs respectively having step, trapezium or triangle shaped profiles. The curves are generally representative of the theoretical or reference profile of the fiber, it being possible for the stresses induced during manufacture of the fiber to give rise to a profile that is significantly different.
To use a fiber in a transmission system, and in particular in a WDM transmission system, it is advantageous for the fiber to have a large effective area in the wavelength range of the multiplex. A large effective area makes it possible to limit power density in the fiber for constant total power, and to limit or to avoid undesirable non-linear effects.
For high data rate systems, it is also useful for the fiber to provide single-mode propagation of the channels of the multiplex. ITU-T G 650 gives a definition of the cutoff wavelength in cable. The theoretical cutoff wavelength of the fiber is generally longer than the cutoff wavelength in cable by several hundred nanometers. The propagation in an optical fiber can be single-mode even if the theoretical cutoff wavelength is greater than the wavelength of the signals used: beyond a distance of a few meters or a few tens of meters, which is short compared with the propagation distances in optical-fiber transmission systems, the secondary modes disappear because of attenuation that is too great. Propagation in the transmission system is then single-mode.
It is also important for the fiber to have sensitivity to bending and to microbending that is as low as possible. Sensitivity to bending is evaluated, as explained in Recommendation ITU-T G.650, by measuring the attenuation caused by winding 100 turns of a fiber about a reel having a radius of 30 mm. Sensitivity to microbending is measured in a manner known per se; it is possible, as below, to measure it relative to a fiber such as the fiber sold by the Applicant under the reference ASMF 200.
In new high data rate WDM transmission systems, it is advantageous to limit the chromatic dispersion gradient within the wavelength range of the multiplex; the aim is to minimize distortion between the channels of the multiplex during transmission.
Dispersion shifted fibers (DSF) have appeared on the market. Those fibers are such that, at the transmission wavelength at which they are used (which in general is different from the 1.3 xcexcm wavelength for which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero, i.e. the non-zero chromatic dispersion of silica is compensated, hence the term xe2x80x9cshiftedxe2x80x9d, by increasing the index difference xcex94n between the core of the fiber and the optical cladding. That index difference makes it possible to shift the wavelength for which chromatic dispersion is zero; it is obtained by inserting dopants into the preform while said preform is being manufactured, e.g. by means of a Modified Chemical vapor Deposition (MCVD) process which is known per se and not described in any more detail herein. The term xe2x80x9cnon-zero dispersion-shifted fibersxe2x80x9d (xe2x80x9cNZ-DSFsxe2x80x9d) is used to designate dispersion-shifted fibers that have non-zero chromatic dispersion for the wavelengths at which they are used. The non-zero value of the chromatic dispersion makes it possible to limit non-linear effects in the fiber, and in particular to limit four-wave mixing between he channels of the multiplex. As explained in EP-A-0 859 247, the problem with DSF fibers is that the chromatic dispersion gradient generally increases with increasing effective area.
EP-A-0 859 247 describes DSF fibers having ring profiles, and explains that, for such fibers, there exists a range in which in the effective area and the chromatic dispersion gradient vary in different directions. The fibers given by way of example have dispersion gradients that are negative and in the range xe2x88x924.5 ps/(nm.km) to 1.0 ps/(nm.km). They have cutoff wavelengths greater than 1500 nm, for a fiber length of 2 m. That document indicates that such a high value for the cutoff wavelength is not problematic insofar as the cutoff wavelength decreases with increasing propagation distance, and insofar as single-mode propagation is provided for transmission distances of about 1000 km.
In xe2x80x9cPractically feasible dispersion flattened fibers produced by VAD techniquexe2x80x9d, ECOC""98 (p.131-132), Y. Yokohama et al propose obtaining an effective area of about 50 xcexcm2 and a chromatic dispersion gradient of about 0.026 ps/(nm2.km) by pushing back the cutoff wavelength beyond 1550 nm.
In xe2x80x9cMaximum effective area for non-zero dispersion-shifted fiberxe2x80x9d, OFC""98 ThK3, P. Nouchi proposes a comparative study of the maximum effective area for various fiber profiles, as a function of bending losses, for fixed dispersion and dispersion gradient values. That article shows, in particular, that, with everything else remaining equal, fibers of coaxial-with-ring profile, or of coaxial profile have larger values of effective area. FIG. 3 of that article shows various types of fiber with the corresponding dispersion gradients; the legend indicates that the chromatic dispersion at 1550 nm is equal to 4 ps/(nm.km). Assuming that the chromatic dispersion is substantially linear around the wavelength xcex0 for which chromatic dispersion is zero, it appears that, in all cases, the xcex0 wavelength is longer than 1488 nm.
Under the name TrueWave/RS, Lucent proposes a fiber having the following characteristics:
xcex0 wavelength: 1468 nm;
chromatic dispersion gradient at 1550 nm: 0.045 ps/nm2.km);
chromatic dispersion at 1550 nm: 3.7 ps/(nm.km);
mode diameter at 1550 nm: 8.4 xcexcm; and
effective area at 1550 nm: 55 xcexcm2.
Under the trademark LEAF, Corning sells NZ-DSF fibers that, at 1550 nm, have an effective area of 72 xcexcm2 and a chromatic dispersion gradient in the range approximately 0.08 ps/(nm2.km) to 0.09 ps/(nm2.km); the chromatic dispersion becomes zero at about 1500 nm.
The invention proposes an optical fiber that is suitable for being put in a cable, and that offers an advantageous compromise between effective area and chromatic dispersion gradient, in particular because of the choice of the cutoff wavelength. More precisely, the invention provides an optical fiber that is single-mode in cable having the following characteristics for a wavelength of 1550 nm:
an effective area greater than or equal to 60 xcexcm2;
chromatic dispersion lying in the range 3 ps/(nm.km) to 14 ps/(nm.km);
a chromatic dispersion gradient in the range 0 ps/(nm2.km) to 0.1 ps/(nm2.km);
a ratio between the effective area and the chromatic dispersion gradient that is greater than 1000 xcexcm2.nm2.km/ps; and
a zero chromatic dispersion wavelength less than or equal to 1480 nm.
Preferably, the fiber of the invention has chromatic dispersion at 1550 nm that lies in the range 5 ps/(nm.km) to 11 ps/(nm.km), and/or a dispersion gradient of less than 0.07 ps/(nm2.km).
The ratio between the effective area and the chromatic dispersion gradient preferably remains less than 5000 xcexcm2.nm2.km/ps.
In an embodiment, the effective area of the fiber is greater than or equal to 70 xcexcm2. 
In another embodiment, the fiber has bending losses at 1550 nm that are less than or equal to 0.05 dB for 100 turns of the fiber about a radius of 30 mm, and preferably less than or equal to 0.005 dB. It may also have sensitivity to microbending of less than 1.2, and preferably less than 0.8.
Preferably, the fiber has a theoretical cutoff wavelength longer than 1550 nm, and an in-cable cutoff wavelength shorter than 1300 nm.
In an embodiment, the fiber has attenuation at 1550 nm of not more than 0.23 dB/km, and polarization mode dispersion of not more than 0.08 ps.kmxe2x88x920.5.
In another embodiment, the fiber has a trapezium-with-ring index profile. In which case, the difference between the index of the central portion of the fiber and the index of the cladding advantageously lies in the range 6xc3x9710xe2x88x923 to 9xc3x9710xe2x88x923, and the difference between the index of the ring and the index of the cladding lies in the range 2xc3x9710xe2x88x923 to 5xc3x9710xe2x88x923. It is also possible to make provision for the fiber to be such that the ratio between the radius of the trapezium and the outside radius of the ring lies in the range 0.42 to 0.58. In addition, the ratio between the inside radius of the ring and the outside radius of the ring advantageously lies in the range 0.68 to 0.85. It is also advantageous for the outside radius of the ring to lie in the range 8 xcexcm to 10.5 xcexcm.
In yet another embodiment, the fiber has a coaxial-with-ring index profile. In which case, the fiber advantageously has one or more of the following characteristics:
a difference between the index of the ring and the index of the cladding that lies in the range 0.5xc3x9710xe2x88x923 to 5xc3x9710xe2x88x923;
a ratio between the inside radius of the ring and the outside radius of the ring that lies in the range 0.65 to 0.85; and
an outside radius of the ring that lies in the range 7.5 xcexcm to 11.5 xcexcm.
Finally, the fiber may have a coaxial index profile with a buried outer portion. In which case, it is advantageous for the outside radius of the outer cladding to lie in the range 7.5 xcexcm to 9 xcexcm.
In the case of coaxial-with-ring or coaxial profiles, the fiber advantageously has one or more of the following characteristics:
a difference between the maximum index of the coaxial portion of the fiber and the index of the cladding that lies in the range 7.2xc3x9710xe2x88x923 to 10.5xc3x9710xe2x88x923;
a difference between the index of the inner cladding and the index of the cladding that lies in the range xe2x88x926.7xc3x9710xe2x88x923 to xe2x88x924.1xc3x9710xe2x88x923; and
a ratio between the radius of the central portion and the radius of the coaxial portion that lies in the range 0.35 to 0.55.
The invention also provides a wavelength division multiplexed optical fiber transmission system including such a fiber as the line fiber. It is then also possible to make provision to include dispersion-compensating fiber.